Resonators are basic building blocks for a number of present and future integrated photonic components, such as switches, Lasers, filters and sensors. For instance, FIG. 1 shows results obtained by simulating wave propagation in a typical prior art ring resonator, characterized by an inside (internal) radius ri, outside (external) radius ro and mean radius rm.
A first issue with such a resonator arises from small evanescent fields they produce, which do not allow for satisfactory electromagnetic coupling to surrounding material. A high refractive index material for the ring is necessary for tight bends and therefore dense integration. Then, however, light is strongly confined in the ring. Only a small fraction thereof is evanescent and allows for coupling to material in the cladding/surrounding (e.g. electro-optic polymers, gain material or to-be-detected material/particles with sensor applications). Furthermore, the peak intensity is confined in the high index material and is not available for coupling to surrounding material. This also makes optical trapping of nanoparticles, biological entities (e.g., cells) very difficult (high power, low efficiency). In FIG. 1, a typical in-plane electric field |Exy| is superimposed to the ring resonator's structure. The gray level used makes that opposite sign values of the field are indistinctly rendered; this will be further discussed later.
State-of-the-art solutions are the following:                For ring resonators with very high Q-factor (i.e., very low losses), no practical solution is known, to the best knowledge of the present inventor;        For ring resonators with moderate Q-factor (typically Q<10000), slotted rings can be used. Yet, slotted rings are technologically challenging (i.e., complicated and expensive manufacturing) and not suitable for many materials, combinations thereof, nanoparticles or combinations with microfluidic sensor systems (narrow slots<100 nm are difficult to fabricate and to fill). FIGS. 2A-B show examples of slotted ring resonators (FIG. 2A: resonator with vertical slot, perpendicular to the mean plane of the structure, FIG. 2B: resonator with horizontal slot. In both cases, the active material is at the center of the ring material).        
Another issue is that the coupling strength critically depends on (e.g., nanometer-scale) distance between the ring and a neighboring waveguide. Namely, it is difficult to reliably achieve critical coupling due to fabrication variations, i.e., width variations of waveguides and/or gap.
The following documents provide details as to the background art in the field:    “Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide”, Bock et al., Optics Express 18, 20251 (2010);    “Interface Device For Performing Mode Transformation in Optical Waveguides”, Cheben et al., US 2008/0193079 A1;    “Subwavelength waveguide grating for mode conversion and light coupling in integrated Optics”, Cheben et al., Optics Express 14, 4695 (2006);    “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers” Cheben et al., Optics Letters 35, 2526 (2010);    “Gradient-index antireflective subwavelength structures for planar waveguide facets”, Schmid et al., Optics Letters 32, 1794 (2007); and    Z. Yu and S. Fan, “Complete optical isolation created by indirect interband photonic transitions”. Nature photonics 3, 91-94, (2009).